A typical wind turbine 10 is shown schematically in FIG. 1 of the drawings. It comprises a tower 11, a nacelle 12 mounted to the top of the tower 11 and a rotor assembly 13 mounted to an upwind end 14 of the nacelle 12.
The rotor assembly 13 comprises blades 15 mounted on a central hollow hub 16. Typically there are three blades but there could be two blades or four or more blades. The blades 15 turn with the hub 16 in a rotor plane to drive a generator within the nacelle 12 to produce electricity.
The nacelle 12 can turn relative to the tower 11, the hub 16 can turn relative to the nacelle 12 and the blades 15 can turn relative to the hub 16. Specifically:                the nacelle 12 turns about a generally vertical yaw axis 17 with respect to the tower 11 to keep the rotor plane substantially orthogonal to the wind direction;        when driven by the wind, the rotor turns about a near-horizontal rotor axis 18 with respect to the nacelle 12; and        to adjust their angle of attack, each blade 15 can be turned with respect to the hub 16 about a pitch axis 19 that extends radially from the rotor axis 18 along a centre line of the blade root 20, when viewed along the rotor axis 18.        
As the blades 15 will bend in a downwind direction under wind forces, measures may be necessary to avoid any risk of the lowest blade of the rotor 13 striking the tower 11 in extremis. Firstly, the rotor axis 18 is typically set a few degrees above the horizontal 21. Secondly, the blades 15 commonly have a negative cone angle with respect to the hub 16 as shown, i.e. the blades 15 are inclined in a slightly upwind direction moving radially outward from the hub 16. The cone angle is exaggerated in FIG. 1, and in reality would typically only be a few degrees at most. Thus, whilst the pitch axis 19 extends radially from the rotor axis 18 when viewed along the rotor axis 18, the pitch axis 19 is not orthogonal to the rotor axis 18: the rotor plane is only approximately planar.
Various annular bearings enable the movements of the nacelle 12, the hub 16 and the blades 15. Specifically:                a yaw bearing between the tower 11 and the nacelle 12 lies in a substantially horizontal plane;        a rotor bearing between the hub 16 and the nacelle 12 lies in a near-vertical plane orthogonal to the rotor axis 18; and        a pitch bearing between each blade root 20 and the hub 16 lies in a plane orthogonal to the pitch axis 19.        
Typically, a generally circular hub plate extends across each blade bearing. The hub plates are primarily stiffening bulkheads of the hub 16 but may also serve as work platforms for technicians working within the stationary hub during installation and maintenance of the wind turbine. In particular, a hub plate may serve as a work platform when the blade 15 associated with that hub plate is parked to hang downwardly from the hub in alignment with the tower. A downwardly-hanging blade 15a is shown in FIG. 2.
It is known in the prior art for each hub plate to lie in a plane parallel to the associated blade bearing and thus orthogonal to the pitch axis 19 (FIG. 1). However, the pitch axis 19 of the downwardly-hanging blade may depart significantly from the vertical: individually and especially in combination, the inclined rotor axis 18 and the cone angle tend to tilt the pitch axis 19 of that downwardly-hanging blade in the upwind direction, away from the tower 11, as shown in FIG. 1. Consequently, a hub plate may be inclined too far from the horizontal to make an ideal work platform.